In recent years, it has been general practice to install security cameras in shops, downtown areas, housing complexes and the like and install vehicle drive recorders or the like in commercial vehicles. The number of cases in which video images are used as material evidence has increased. A store typically records and maintains a conversation carried out between an operator of the store and a customer as evidence including in case any problem arises later from discussion over the telephone or in relation to subsequent support operations.
If a video image or sound is used as evidence, a video tape, an image and sound file are directly submitted as it is. If the digital technique handing the image and sound storage advances further, editing and alteration are easy to do. To use the video image as evidence, the data needs to be endorsed with a digital signature or a time stamp by a third party. Currently, services and products are commercially available to record the voice of an operator speaking on the telephone with a time stamp attached thereto. The need for such a technique is expected to increase from in the future.
A typical technique of detecting any alternation by a third party is to split the content of a digital document according to items, determine summary information of each item, and attach a digital signature to the set of summary information of each item. The summary information, also referred to as a message digest, is hash information that is calculated using a cryptographic one-way hash function. This technique is applied to video image data such that originality of the video image data is assured, and such that data is extracted from a signed document with privacy protected (for example, Japanese Laid-Open Patent Publication No. 2008-178048).
Since video data is typically large in size, a variety of compression techniques are used. A typical interframe prediction technique is available as one of the compression techniques. For example, the interframe prediction technique is used in the motion picture expert group (MPEG)-1 format of compression video data. Three types of images, I picture, P picture, and B picture are stored. The I picture includes all images needed to display video. The P picture includes a difference between the past I and P pictures. The B picture includes a difference between past/future I pictures and the P picture. Since the P and B pictures include differences between prior images and subsequent images, a high data compression rate is achieved.
A large amount of process is needed to decode video data based on the interframe prediction technique. A frame as the I picture is thus extracted, and encoded into a still image on a per frame basis. The video data is thus played quickly (as described in Japanese Laid-Open Patent Publication No. 2006-74690).